Chromium-plated steel



Dec. 10, 1963 HIROMU UCHIDA ETAL 3,113,845

CHROMIUM-PLATED STEEL Filed March 21. 1961 on 3 2am:

EFFICIENCY 0F ELECTRIC CURRENTIZ) INVENTOR. Uc m 0a osnMu YmmEw 711% M MUnited States Patent s,11s,s4s CHROMEUM-PLATED STEEL Hirolnu Uchida and@sainu Yanahu, l-lyogo-iren, Japan, assignors to Fuji iron 5; Elteel Co,Ltd, Tokyo, Japan Filed Mar. 21, 1961, Ser. No. 97,25 3 Claims priority,application lapan Mar. 29, 1960 3 Qlainrs. (*Cl. 2Q---l%3.5)

This invention relates in general to chromium-plated steel adapted toreceive surface coatings, and more particularly, to chromium-platedsteel sheet adapted to receive surface coatings, such as paint or thinlayers of synthetic resin and to a method for making these materials.

The usual chromium plating applied to steel sheets has been applied toshaped steel sheets with the object rendering the steel sheetanti-corrosive or for ornamental purposes. become hard by the absorptionof hydrogen produced thereon. However, pin holes will be caused by thebubbles of hydrogen. In an effort to avoid this in the conventionalpractice, the chromium of 0.254 has been deposited on copper, nickel ortheir double plating to be used but mainly for ornamental oranti-corrosive purpose. When the chromium is plated directly on thesteel sheet, it has poor anti-corrosive properties unless it is madefairly thick. The thickness of the chromium-plated layer must be mademore than 3 1. to possess sufficient anti-corrosive power. However,these materials are still troubled with pin holes and are not workableafter plating. Moreover, it has suificient anticorrosive power, so thatit has been used without further anti-corrosion coating.

A primary object of the invention is to provide an improvedchromium-plated steel.

It is a further object of the present invention to provide a chromiumplated steel material which has high adhesive and workabilitycharacteristics and which is adapted to readily receive and retainsurface coatings so as to give it good anti-corrosion characteristicsand at the same time to avoid the diiliculties of the prior art.

Other and more specific objects, features and advantages of theinvention will appear from the detailed description given below iiiconjunction with accompanying drawings which form a part of thisspecification and illustrate by way of examples preferred embodiments ofthe invention. In the drawings: FIG. 1 is a schematic elevation of anembodiment of the invention, showing a working process thereof. FIG. 2,is a diagram, illustrating the relation between efficiency of electriccurrent and time of plating.

Several methods such as chemical treating, metal plating, etc. have beenproposed to give steel sheet desirable properties such as anti-corrosivepower, workability, etc. However, the method of the present invention,although involving a plating operation, is entirely different from themetal plating method included in the prior art. Basically, it involvesplating a very thin chromium layer (within the limits of specifiedthickness) directly on the surface of a steel sheet. The surface ispreferably precleaned by usual treatment. A surface coating is thenapplied to the chromium layer. As a result of these steps, a sheet willbe made which is provided with such desirable properties as adhesion forpaints, anti-corrosive power, workabilities for bending, drawing, etc.,heat resistance for baking of coating, etc., and a fine surface which isadapted for support of coatings on polished steel sheet. As comparedwith other chemically treated steel sheets or steel sheets plated withtin, zinc, etc. in the prior art, the present products are decidedimprovements.

In the method of chromium plating, according to the present invention,ordinary carbon steel sheet or strip In general, the chromium-platedlayer will Patented Dec. HE, 363

(band) steel is treated with usual surface cleaning agents. While movingthe steel sheet a chromium layer of 0.1 to 0.00% thick is continuouslyplated thereon, the thickness depending on the method employed to effectchromium plating. The chromium plating of this thickness and surfacecondition is such as to give the optimum surface for steel sheet to becoated with paint.

The chromium plated steel sheet, per se, is excellent in its adhesion topaints, in its resisting power against acid, alkali, weather and saltwater, and in its thermostability upon the baking of paints. Moreover,its color and luster are as beautiful as those of tin plate surfaces. Incontrast, other chemically treated steel sheets have gray white color orfluorescence, etc. Furthermore, its luster is not lost when a paintlayer is coated thereon. When coated with paint as mentioned below, thesteel sheet is given further desirable characteristics such asanti-corrosive power and workabilities in addition to the characteristicobtained by chromium plating thereon.

According to this invention, the chromium plating is made in ahomogeneous layer by controlling the composition of plating bath,temperature of plating liquid,

density of electric current and time of plating, so as to minimize pinholes formation and to avoid the generation of hydrogen.

Moreover, the proper control of these factors avoids the need for arocker in the plating operation, avoids the generation of stripes due tothe movement of non-homogeneous plating liquid etc. and avoidsinferiority luster in the product. As the occasion demands, for thepurpose of improving the anti-corrosive power to preserve or store theplated steel so that it will maintain its proper characteristics withindesired limits, it is effective to employ an auxiliary treatment using23% solution of Na Cr O or about 1% solution of CrO In the continuousplating of strip steel, it is preferred to use a single bath. However,the numbers of tanks and baths are determined by the particularcircumstances. The chromium plated strip steel obtained in accordancewith this invention may be preserved for a long time for reception ofpaint, by daubing it with vegetable oil such as cottonseed oil or wax asanti-corrosive oil, or may be painted as it is.

When the chromium plated surface of this invention is furtherelectrically plated with zinc, a steel sheet for outdoor use, which isparticularly excellent in resistance against weather is obtained.Moreover, a surface treated steel sheet, having very good workabilityadapted for deep drawing is thus also obtained.

The anti-corrosive power of a paint coated chromium plated steel sheetaccording to the invention will be described below.

It is generally considered that the anticorrosive power of chromiumplated steel sheet will be decreased regardless of the thickness ofplating when it is subjected to bending or drawing unless it is coatedwith paint. Even if only spots are coated, the plated layer will beextended, so that it is usually regarded that the thinner the plating,the lower will the anti-corrosive power be decreased during 'working.

in contrast to this, according to the present invention, bothworkability and anti-corrosive power of the chromium plated steel sheetwill not be decreased in the least when the thickness range ofthepla-ting is in the range of about 0.1 to 0.005;. It has been foundthat both workability and anti-corrosive power thereof are superior tothat of tin plate which is coated with the same paints in the manner andis drawn in the same method.

In the production of chromium plated steel sheet according to theinvention, as shown in FIG. 1 strip (band) steel is moved and oil andfats on its surface are continuously removed in an electrolyticaldeoiling tank 1. After washing, it is dipped in a pickling tank 2 toremove rust on its surface. Then it is washed and electric plating iseffected with chromium in a chromium plating tank 3. After washing withhot water, it is chemically treated with about 1% chromic acid liquid ina chromic acid treating tank 4, then it is washed with hot water andoiled with emulsion oil by means of oiler 5. It is squeezed by a pair ofrolls 6 and dried in a dryer 7, thus the whole working process will befinished.

The chromium plating tank may be either the horizontal type or thevertical type. The negative electrode will be electrically connected tothe strip steel through charging rollers and positive electrode will beelectrically connected through the lead anodes and the plating liquid iscontinuously circulated at a constant speed in the tank. In FIG. 1, 8 isan uncoiler, 9 is a recoiler, 16* is a pair of pinch rollers, 11 is awelder, 12 is a tension bridle, 13 are spray washers, 14 are hot waterwashing tanks and 15 is a drive bridle.

In case the plating tank is increased to two or three units, the stripsteel will move from one tank to another. When the strip steel istemporarily exposed in the air, thus interrupting the contact betweenthe surface of strip steel and the plating liquid, discontinuous platingwill result with a loss of luster. When a lustrous appearance is desiredto be secured on both sides of a strip steel, in

vertical and horizontal type plating tanks, it is possible to obtaincontinuously lustrous plating by dipping both surfaces of the stripsteel continuously in the plating liquid.

In order to obtain the thickness of plating between about 0.005 to 0.1the conditions for plating are as follows:

It has been found that there is a relation between the time of platingand the efficiency of electric current (as shown in FIG. 2), whentemperature and density of electric current is held constant in variouskinds of chromium plating liquid. According to this relationship, thebest efficiency of electric current will be obtained at about 10 secondsof plating time, when the plating is effected under In actual operation,when chromium plating is effected by using a lead anode, if the platingis discontinuously effected by cutting of? electric current after acertain time of plating and then further effecting the plating after acertain interval of time, the surface of lead anode will be connected tothe lead salt of chromic acid. In such a case, when electricity isconveyed thereto, polarization is caused, thus making the chargingdifiicult. These troubles may be prevented by applying reverse electriccurrent for a few minutes by making the lead anode negative and thestrip steel positive, when the plating is finished.

Examples of the test relation to steel plate of 0.27 mm. are shown asfollows:

Degree of drawing is shown in Er-ichsen values. If the value is mademore than 7 m./m., the steel sheet, per se, is feared to be broken. TheErichsen value of this steel sheet is 78 m./m. In the test, chromiumplated steel sheets according to the present invention are coated withphenol resin paint of about 8 and then they are dipped in ferro-oxyltesting liquid for 30 minutes. In the Table 1, A means that the convexportion is all right, B means that the convex portion has spotcorrosions, C means that the convex portion has many spot corrosions andD means that the whole surface has spot corrosions.

the same conditions. Therefore, in the present invention, Th6Composition 0f feYTO-OXYI testing liquid is! the plating will beeffected continuously in the range of lr (C )s' z 0% g-/ 3 (CN) 10 1-20seconds, while the strip steel is moving. g./l. and NaCl g./l.

TABLE 2 Citric acid Sulfuric acid Exposure proof test proof test; Saltwater Soy-proof test (exposed Adhesive (boiling in (dipping in sprayingtest test (dipping on the roof- Itcnls of test power of 1% citric 5%H250 (for 48 hr. in soy for top for 10 days paints acid solusolution forat normal 5 days at (including tion 5 hr.) 48 hr. at nortemp.) 75 C.) 2rainy mal temp.) day Chromium plated steel plate of the invention:

Tllizkness of plating,

G A B A A B (D A B A A B O B B B A B O B B B A B Q B B B A B G O B B A B9 D C O B C Q D D D I) 0 Electric tin-plate O B C B B B Plating a stripsteel within the range of 1-20 seconds to make a finished product hasnever been shown in the prior art. The speed-up of plating under theseconditions brings great advantages to the industrial manufacturingtechnique.

As accompanying conditions under which the chromium plating is effected,it is preferred that the density of electric current must be taken as2060 A./dm. that is high density of electric current for high speedplating. The temperature must be preferably between 30-50 C., theconcentration must be preferably between 100 250 g./l. as CrO andfurther the addition agent such as sulfuric acid must be preferablyabout of CrO in weight.

This is generally used as a paint for food can, and is mixed phenolformaldehyde resin and vinylbutyral resin with some solvent.

This is generally used as a paint for food can, and is reformed fromphenolic resin by addition of tung oil. a

D means that all surface corroded and B means that the ring part ofErichsen rusted.

The results shown in Table 2 show that the chromium plated steel platesaccording to the invention are superior to the tin plate painted underthe same conditions, and also the sulfuric acid proof power of theformer is shown to be better than that of the latter. Therefore, in caseof canned food, the chromium plated steel sheets according to theinvention will be used without the apprehension of sulfurstain, and incase of canned beer, there is no fear that the beer may become cloudy,so that the steel sheet will be used more advantageously than tin-plate.

As seen from the above description, in the chromium plating, when thethickness thereof becomes more than 0.1 4, the workability of the sheetwill decrease as shown in Table 1, so that anti-corrosive power afterworking thereof will become inferior, and when the thickness thereofbecomes less than 0005 both workability and anti-corrosive power of thesheet become decreased as shown in Table 2. Thus, these thicknessesbecome the base of limiting conditions for chromium plating of theinvention.

Further, the comparison between a chromium plated steel sheet of0.1-0.005 thick and electrical tin-plate, regarding their adhesivepowers for paints, will be shown as follows:

'(1) The method for testing.

Drop adhesion test is adopted to investigate the adhesive power ofpaints. A test piece is placed horizontally, on which 3-5 drops of paintare laid. When the paint spreads in circular shape, it is baked at 210C. for ten minutes, then cooled to room temperature, and cut by a needleto mark X. The degree of exfoliation is inspected by a cellophane tapeand judged by the naked eye from a square root of percentage ofexfoliated area of the paint, and it is defined so that no exfoliationis grade and 100% exfoliation is grade, respectively.

Based on the above method, tests were made with chromium plated steelplates and electric tin-plates, which were lain in a tank of constanttemperature and humidity (40 C., 97%) for 20 hours, 40 hours, one weekand three weeks, respectively. As the paint No. 9635 (Phenolresin madeby Fuller Co.) was used, the values for electric tin-plates are resultsof the best chemical condition in the operation.

TABLE 3 Electric Chrominium The time clasped, hours tin-plates, platedsteel grade sheet, grade From the above table, it will be understood howlong the adhesive power of the paint is effective for the products ofthe invention.

One of the reasons why the surface of chromium plated steel sheet ofthis invention and the paint have very good adhesive and anti-corrosivepowers for long periods of time is thought to be caused by theanti-corrosive power of said surface. The reason thereof is not clear,but it is believed that even if some corrosive substances permeateperpendicularly to the sheet through the paint layer, the corrosion willnever spread horizontally, so that any reaction between the steel baseand said substance will not be accelerated by such a degree of scratchesor pin holes.

As paints to be used for the chromium plated steel sheet according tothe invention are, by way of example,

phenol based paints, oil based paints, vinyl-resin based paints,epon-series paints, varnish, lacquer, etc., and these paints are usedsingly or compound. As synthetic resin film, various kinds such asvinyl-series resin, phenolicseries resin, etc. are suitably used with orwithout binding agent. Particularly, when used for cans to be employedfor canned food, it is not necessary to select paints according to thespecies of canned food as it necessary with tinplate, yet very goodanti-corrosive power will be assured at all times.

Example 1 A polished steel sheet of 0.27 m./m. thick waselectrolytically de'oiled in de-oiling liquid containing 5% causticsoda, 3% sodium silicate and 0.2% non-ion intersurface activator at C.,with current density of 6 A./dm. for 20 seconds, and washed by coldwater. Then the steel sheet was used as a cathode in electrolytecontaining 250 g./l. of chromic acid anhydride and 2.5 g./l. of sulfuricacid and was electrolysed at 50 C., with current density of 20 A./dm.for 10 seconds, forming chromium plating layer of 0.05 thick on thesteel sheet.

Chromium plated steel sheet thus obtained had very good adhesive powerto various kinds of paints, and when dipped in 5% sulfuric acid, itdisplayed an excellent anticorrosive power and as was unaffected for 168hours (contrary to other kinds of plated steels which were corrodedabout 5-48 hours). Its citric acid proof power was found to be about thesame as tin-plate. In alkali proof test, it was unaffected for 144hours. Similarly, in the Hinac test, it showed excellent conditions. Incontrast, to this, other kinds of plated sheets lasted only for about24-48 hours.

In chemical treatment, the chromium plated steel sheet had similaranti-corrosive power to the plate according to the rI-Iinac method, andwhen anti-corrosive oil was applied to the former, there was found nochange for 580 hours.

The color and luster were as beautiful as the tin-plate and adapted forgrounding treatment. Various test examples of the chromium plated steelsheets obtained which were coated with phenol-resin based paint layer of48/L thick are shown in the Table l and Table 2.

A strip steel of 0.27 m./rn. thick was preliminary treated under thesame conditions as Example 1, then it was plated with a layer of 0.005thick of chromium in an electrolyte containing g./l. of CrO and 1.5g./l. of H 80 at 40 C. with current density of 20 A./drn. for 1 second.

Various test examples of thus obtained chromium plated strip steel whichwere coated with phenol-resin series paint layer of about 8 thick areshown in Table 1 and Table 2.

Example 4 A cold rolled steel sheet of 0.9 m./m. thick was preliminarytreated under the same conditions as mentioned in Example 1, then it wasplated with chromium layer of 0.1 thick in an electrolyte containing 150g./l. of CrO and 1.5 g./l. of H 80 at 40 C., with current density of 20A./dm. for 20 seconds, and then the plated surface was applied withvinyl series paints of about 15 thick to make a final product.

Although some preferred embodiments of the invention are hereindisclosed for purpose of explanation, various modifications thereof,after study of this specification, will be apparent to those skilled inthe art to which the invention pertains. Reference should, accordingly,be had to the appended claims in determining the scope of the invention.

What is claimed is:

1. A method of producing can stock receptive to organic surfacecoatings, which comprises continuously moving a thin-gauge steel sheetthrough a chromium plating bath for a period of between about 1 to 20seconds, thereby to plate said steel sheet with a chromium layer of asubstantially uniform thickness of about between 0.04 to 0.1 1,immersing the chromium plated sheet in a chromate containing bath,Washing the sheet with Water and finally coating the sheet with asurface layer of an organic coating composition, said organic coatingcomposition being selected from the group consisting of phenol basedpaints, oil based paints, vinyl-resin based paints, epon based paints,varnishes and lacquers.

2. Can stock essentially consisting of a thin-gauge steel sheet having achromium layer of a uniform thickness of from about between 0.04 to0.10, directly plated thereon, said chromium layer being surface coatedwith a layer of a coating composition selected from the group consistingof phenol based paints, oil based paints, vinylresin based paints, eponbased paints, varnishes and lacquers.

3. As an article of manufacture a surface coated thingage steel sheetcomprising a single sheet of steel having directly plated thereon achromium layer having a thickness of from 0.005 1. to 0.1,, saidchromium layer being surface coated With a coating composition selectedfrom the group consisting of phenol based paints, oil based paints,vinyl-resin based paints, epon based paints, varnishes and lacquers.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Morisset et al.: Chromium Plating published by Robert DraperLimited, 1954, pages 50-53.

1. A METHOD OF PRODUCING CAN STOCK RECEPTIVE TO ORGANIC SURFACE COATINGS, WHICH COMPRISES CONTINUOUSLY MOVING A THIN-GAUGE STEEL SHEET THROUGH A CHROMIUM PLATING BATH FOR A PERIOD OF BETWEEN ABOUT 1 TO 20 SECONDS, THEREBY TO PLATE SAID STEEL SHEET WITH A CHROMIUM LAYER OF A SUBSTANTIALLY UNIFORM THICKNESS OF ABOUT BETWEEN 0.04 TO 0.1U, IMMERSING THE CHROMIUM PLATED SHEET IN A CHROMATE CONTAINING BATH, WASHING THE SHEET WITH WATER AND FINALLY COATING THE SHEET WITH A SURFACE LAYER OF AN ORGANIC COATING COMPOSITION, SAID ORGANIC COATING COMPOSITION BEING SELECTED FROM THE GROUP CONSISTING OF PHENOL BASED PAINTS, OIL BASED PAINTS, VINYL-RESIN BASED PAINTS, EPON BASED PAINTS, VARNISHES AND LACQUERS. 